One-trillionth (pico) is one-millionth of one-millionth. This is the same as micro-micro, which is why you will sometimes see “mmf” instead of “pf.”

To convert micros to picos, you multiply by 1,000,000 because there are 1,000,000 picos in each micro.

Examples

.00047 mf * 1,000,000 = 470 pf .001 mf * 1,000,000 = 1000 pf

To convert picos to micros, you divide by 1,000,000 because each pico is one-millionth of a micro. For example:

470 pf / 1,000,000 = .00047 mf 1000 pf / 1,000,000 = .001 mf

CAPACITOR SUBSTITUTIONS AND TONE

Using a larger capacitance allows lower frequencies to get through, thus darkening the sound. Using smaller capacitances make it harder for lower frequencies to get through, thus brightening the sound.

In the Marshall amp example that you mentioned (page 595 Tube Amp Book IV), a 470 pf cap is parallel to a 470K resisitor. The 470K resistor is in series with a 1M resistor. At their intersection (2) is the .001 mf cap, which couples the next stage of the preamp (4):

The 470K and 1M resistors are a voltage divider. A lower frequency signal cannot easily make it through the 470 pf capacitor, so it flows through both the 470K and 1M resistors. By the time the signal gets to (3), it is at ground potential (0). At (2) it is somewhere between its maximum voltage at (1) and ground. The voltage that appears at (2) is what goes through the .001 mf cap and gets amplified by the stage at (4).

A higher frequency signal can sneak through the 470 pf cap, bypassing the 470K resistor. Its full voltage appears at (2), goes through the .001 cap, and is amplified by the next stage. So the higher frequency signal gets amplified more than the lower frequency signal.

If you increased the value of the 470 pf cap (or removed it) you would darken the sound of the amp. If you decreased it, you brighten the sound of the amp.

The .001 mf coupling cap affects all signals going to the stage at (4). If you increased the value of this cap, say to .01, the amp would sound darker. If you decreased, say to 470 pf, the amp would sound brighter. Both the 470 pf and .001 mf caps need to be rated for high voltages.